Frigatebirds May Sleep in the Air

Migrating birds burn up reserves even though they feed at regular pitstops during their flight over oceans. So what energy-saving trick do great frigatebirds use?

A juvenile frigatebird. Credit: Henri Weimerskirch, CEBC CNRS

A juvenile frigatebird. Credit: Henri Weimerskirch, CEBC CNRS

Animals do the most amazing things. Read about them in this series by Janaki Lenin.

Great frigatebirds are birds of the open oceans. They wheel around in the skies for days, riding thermals and keeping a sharp eye out for prey below.

Flying confers an advantage on birds. They don’t have to climb trees and mountains, wade across rivers or oceans. They flap their wings to create lift and fly over barriers that frustrate land and sea animals. But this ability to get airborne comes at a cost. Flying is a carbon spewer for humans and energy guzzler for birds. Migrating birds burn up reserves even though they feed at regular pitstops along the way. Some lose a quarter to half their body weight in making these long distance journeys. So what energy-saving trick do great frigatebirds use to soar across oceans?

The metre-long black birds are built like ultralight aircraft – their small bodies, weighing less than 1.5 kilograms, have a wingspan of more than two metres. Their ratio of wing reach to body weight is greater than any other bird. Their long, narrow, pointed wings are ideal for soaring high, and they seem to prefer staying aloft even when land is in sight. Biologists know a lot about their breeding behaviour on land but have little idea what goes on at sea.

Scientists from France, Britain, Canada and Germany went to work at Europa Island. The team was led by Henri Weimerskirch, an ecologist at Centre d’Etudes Biologiques de Chizé, France. The island that lies between mainland Africa and Madagascar has a large colony of great frigatebirds. The researchers caught birds using nooses at the end of telescopic poles by day and by hand with the aid of night vision goggles at night. They outfitted 24 adults and 25 juveniles with solar-powered satellite transmitters, while eleven birds also got data loggers to measure heart rate, wing flaps and speed of flight. With these gadgets, the researchers tracked the birds’ movements for four years.

Between June and September, frigatebirds with no parental duties left Europa. They caught the trade winds north to the equator and banked eastward toward Indonesia.

Frigatebirds are known to ride the thermals without working their wings much. However, the skies above warm equatorial waters are still, with hardly any wind. Called the doldrums, they were the bane of pre-industrial era sailors. The birds avoided getting stuck in these windless sections and rode the trade winds sweeping from the southern and northern hemispheres that skirted the doldrums.

Rising columns of warm humid air condense to form cumulus clouds. The birds caught this air current, and without flapping their wings, they spiralled effortlessly upward to 700 metres, to the base of the clouds. While they usually coasted horizontally from these heights, they frequently didn’t stop climbing at this point. They continued their corkscrew-like flight pattern inside these clouds to reach more than 4,000 metres. Once frigatebirds reached a peak, they cruised down.

Cumulus clouds suck air with increasing velocity. The taller the clouds, the greater the speed. Paragliders and hand gliders have a difficult time getting out of its hold. How then do frigatebirds fare?

Henri Weimerskirch with a juvenile frigatebird. Credit: Henri Weimerskirch, CEBC CNRS

Henri Weimerskirch with a juvenile frigatebird. Credit: Henri Weimerskirch, CEBC CNRS

“When entering into the cloud, where updraft is much stronger, they do no not flap their wings but climbing rates are three times higher,” Weimerskirch told The Wire. “This shows they are literally sucked in. Turbulence is probably not strong inside the clouds, but when the bird has to come out of the cloud, they have to flap a lot.”

Any activity at high altitudes is laboured because of low oxygen levels. But the birds’ heartbeats didn’t thud fast from the effort of gaining altitude. Since they let the air currents do their work for them, they didn’t spend much energy. So the cost of flying tens of thousands of kilometres was low.

One stayed airborne for two months. Normally, the birds coasted non-stop for up to 48 days, traveling an average of 420 km a day. They rested for 8 to 48 hours in the islands of Indonesia, Seychelles or the Chagos that lie south of Maldives before resuming their great transoceanic flights.

During the day, the frigatebirds flew just above the sea to catch fish. However, they suffer from a critical design fault: their feathers are not waterproof. Should they land on water, they’d get waterlogged and cannot take off. Neither can they dive into the water after fish. By necessity, they catch prey on the surface like flying fish and flying squid that they gulp down in midair. These feeding bouts sap energy because the birds flap their wings vigorously to gain altitude before shooting down to pick prey off the water. Feeding takes no more than 10% of the their time.

Such aerial prowess takes time to master. Parents spend up to two years raising their chicks. This is far longer than any other bird. Once the offspring become independent, they leave their island home and circle over the Indian Ocean, staying airborne for months. One youngster flew large sweeps over the the Indian Ocean, clocking 55,000 kilometres in six months. In all that time, it rested on islets for only four days.

If frigatebirds spend months in the air, even catching and eating their prey on the wing, when do they sleep? Sleep is necessary to restore physiological functioning of the brain. Frigatebirds cannot land on the water like albatrosses and go to sleep. The researchers suggest the birds must sleep in flight, too. As they catch an updraft and soar, they may be able to doze for no more than 12 minutes at a time.

Researchers tracked three alpine swifts that flew non-stop for more than 6 months. They slept as they glided and didn’t seem to suffer from sleep deprivation. More than deep REM sleep that would require muscle relaxation, they could instead go into slow-wave sleep that shuts down one or both hemispheres of the brain at a time while allowing them to keep their wings outstretched.

A juvenile great frigatebird that fledged from Europa travelled over the northern Indian Ocean over six months.

A juvenile great frigatebird that fledged from Europa travelled over the northern Indian Ocean over six months.

The study didn’t provide unequivocal evidence that frigatebirds sleep in the skies. The birds don’t forage at night and the lack of any sign of activity at night indicates they may be asleep.

Their body design leaves great frigatebirds with no choice but to keep flying. But their adaptations to power this lifestyle are remarkable. This study is the first to track the species on its long-haul flights. As a next step, Weimerskirch says, “We are comparing the influence of environmental and atmospheric conditions on migration strategies of young frigatebirds throughout their range, i.e. in the Galapagos, south west Pacific and Indian Ocean.”

The study was published in the journal Science on July 1, 2016.

Janaki Lenin is the author of My Husband and Other Animals. She lives in a forest with snake-man Rom Whitaker and tweets at @janakilenin.